Sensei

Sensei (先生) is the human who created the Servamps. He was killed by Sleepy Ash hundreds of years ago before the series.

Appearance
While his appearance in the manga hasn't been shown, in the anime, he closely resembles Kuro, having short bluish white hair, wearing white shirt, grey pants, a pair of black boots, and white haori over his body.

Background
Sensei was a researcher born hundreds of years ago. So far, his only known research is about vampires. This research resulted him capable of changing seven humans into a special vampires known as Servamps that each represents the seven deadly sins. However, not all Servamps were happy with their change, particularly Sleepy Ash of Sloth who despite having a close relationship with Sensei, views the existence of monsters such as vampires are unnecessary, and All of Love of Lust who view his change as a punishment since he doesn't wish to live anymore yet was turned into an immortal being.

Despite being a human, he was able to live for a hundred years without aging. During this time, he continued his research in Japan and then changed another human into a Servamp named Tsubaki who represents Melancholy. Since the other Servamps doesn't know of their existence prior their meeting with Tsubaki, it's presumed that Sensei never told any of them about their youngest sibling and kept his existence hidden from them. Furthermore, unlike the other Servamps, Tsubaki was created with the ability to break the contract item that constitutes the link between Servamps and Eves. One day, a neutral organization, C3, sent a request to the seven Servamps, which was to kill their creator. The opinions between the Servamps were divided, so they decided to do a majority vote. The Mother of Wrath, Lawless of Greed, and World End of Gluttony objected due to their attachment to their creator, while Old Child of Pride, Doubt Doubt of Envy, and All of Love agreed out of fear that their creator may create more monsters like them in the future. Sleepy Ash, who held the final vote, also thinks that their creator's research is too dangerous and has to be stopped, so he decided to personally kill their creator. Arriving in Japan, Sleepy Ash confronted Sensei in his house, and Sensei greeted him warmly. Despite his uncertainty, Sleepy Ash still sticked with his decision and killed his creator immediately after he arrived.

Sensei's death left a great impact on Sleepy Ash who soon regretted his action afterwards. Since then, Tsubaki determined to fulfil Sensei's expectation to break the other Servamps and fix their messed up world. Whether Sensei really requested Tsubaki to do so or not is not yet confirmed.

Sleepy Ash
The Creator of the Servamps is the one who made Sleepy Ash a Servamp. Judging from what the other Servamps said, the creator was supposed to be a special person for Sleepy Ash, implying that Sleepy Ash and the creator used to be close.

At some point in the past, Sleepy Ash was approached by with the decision to kill a certain person, the creator of the servamps by the C3. He called his remaining siblings to a family meeting to discuss whether they should accept the request or not. The discussion became an argument so they decided to do a majority vote, with Sleepy Ash making the final decision. Believing that the experiment that their creator was doing was too dangerous and to prevent another "monsters" like them to be born, Sleepy Ash decided that they should kill their creator and he willingly volunteered to be the one to do it. This decision resulted in Kuro having a falling out with Lawless, straining their relationship for years. As per request, Sleepy Ash went to Japan to do the job. Upon meeting his creator for the first time after hundreds of years, Sleepy Ash was surprised at the fact that the information was true, having a hard time believing that his creator is still alive.

However, when before he killed the creator, Sleepy Ash was hesitant and then crying as he killed him. This left a feeling of uncertainty within Sleepy Ash, and soon became regret that continued to haunt him for hundreds of years afterwards, making him questioning his own action. This shows that despite thinking that he is dangerous, Sleepy Ash actually still cares for his creator and regretted for immediately choosing to kill him rather than speaking to him first.

Old Child
Old Child and his siblings received request from C3 to kill their creator. Since Sleepy Ash wanted to hear all of the siblings' opinion, he approached Lawless who was depressed after losing his Eve, prompting Old Child to remind Lawless of his warning before about not to fall in love. He then gave the letter where all of them will hold the meeting.

At the meeting, together with All of Love and Doubt Doubt, he agreed that the creator of the Servamps must be killed. This caused an argument with Lawless, World End, and The Mother, who were opposed of the request. When Lawless stated that their creator's life shouldn't be decided by a simple thing such as a majority vote, Old Child told Lawless that he mustn't include his personal feeling in this matter and questioned that the reason he said so was because of what recently happened to his Eve. Old Child then asked Sleepy Ash who hasn't given his vote what his decision is, which Sleepy Ash answered that he will kill their creator.

After their creator was killed, Old Child stated there's nothing to regret because it was something they had to do.

Doubt Doubt
At the Servamp meeting to decide whether they should kill the creator or not, Doubt Doubt voted for accepting the request. He was concerned with Sensei possibly making more Servamps and wished to settle things calmly.

The Mother
During the meeting regarding C3's request to kill the creator of the Servamps, she was one of Servamp who objected the request, along with Lawless and World End. She stated that while she theoretically understood the benefit of the request, her emotions don't permit it.

Due to the difference in opinions, they decided it with a majority vote, which resulted in Sleepy Ash killing their creator. After the job was done, The Mother stated she's not mad because they don't have the right to be mad.

Lawless
Lawless had a really good relationship with the man who created the Servamps - he describes him as a father figure.

Everything change for Lawless when C3 ordered the Servamps to kill the man who had created them. To decide on the controversial matter, the Servamps decided to hold a survey to determine the fate of their creator.

Lawless having just lost Ophelia to humanity and fearing losing the last piece of human family he had left, Lawless objected to the notion. With the vote split three to three, the oldest Servamp, Sleepy Ash, was the deciding factor. He reluctantly agreed that it was best that the creator be killed and was sent to finish the job himself.

This decision was enough to cause a one-sided dispute between Lawless and his brother for years to come until Licht appears and help him to let go of chasing after Sensei's shadows, finally coming to terms with Sensei's death while still remembering the man who created the Servamps.

World End
Sensei's relationship with World End is mostly unknown. When all the Servamps gathered to decide if they should kill their creator, World End voted against it, along with Lawless and The Mother, implying that World End cared for the creator enough to object the request.

All of Love
It's strongly implied that All of Love used to be a human (like the other Servamps) before he meet the creator of the Servamps. When he was young, he didn't wish to live any longer, but was then turned into a vampire by the creator of the Servamps. All of Love viewed his change into a vampire as a punishment and questioning why his creator did that.

Hundreds of years later, the Servamps received a request from C3 that asked them to kill their own creator. Viewing his creator as being too dangerous, Lily voted for accepting the request, together with Old Child and Doubt Doubt. After Kuro had done the request, Lily commented that this was the best thing to do.

Tsubaki
Tsubaki seems to have a close relationship with the creator of the Servamps that he refers to as "Sensei".

One night, Tsubaki witnessed Sleepy Ash in his black lion form after he killed their creator, and Tsubaki seemed devastated at his creator's death. Since then, he has been looking for the real identity of the black lion to question why he killed their creator.

Since then, Tsubaki determined to fulfil Sensei's expectation to break the other Servamps and fix their messed up world. It's not yet confirmed if Sensei really requested Tsubaki to do so or not.

Sensei's death resulted in Tsubaki's one-sided hatred against Kuro for killing the creator of the Servamps and not knowing him being his youngest brother like his other siblings.

Trivia

 * In Japanese, Sensei (先生) means master or teacher
 * It is unknown if Sensei and Sleepy Ash/Kuro are relative. Since Sensei had short bluish white hair and closely resembles Kuro, it is possible that Sensei and Sleepy Ash/Kuro are relative.